Spinal arthroplasty is an emerging field that offers the promise of restoring and/or maintaining normal spinal motion. The goal of spinal arthroplasty is to reduce or eliminate adjacent segment disease (ASD) by maintaining the normal spinal biomechanics at the operative level. To accomplish this, an artificial cervical prosthesis must duplicate as closely as possible the natural spinal biomechanics, including maintaining the axial height of the disc as well as applying angular adjustment throughout the full range of motion of the natural spine.
The spine plays an integral role in neural protection, load bearing and motion. The vertebral column provides a strong, yet mobile central axis for the skeleton and is composed of twenty-four vertebral bodies with seventy-five stable articulations. The intervertebral disc is a fundamental component of the spinal motion segment, providing cushioning and flexibility. Adjacent vertebrae are linked together by three articulations: a) the vertebral bodies and disc, which transmit compressive and shear loads and provide flexibility, and b) by two facet joints, which protect the disc from translational shear stress and limit rotation. This “triple joint complex” allows for flexion, extension, lateral bending and rotation of the spine.
The intervertebral disc is composed of an inner gel-like matrix called the nucleus pulposus and an outer surrounding fibrous band called the annulus fibrosus. When compressive loads are placed on the spine, increased pressure in the nucleus pulposus is transmitted to the annulus, which bulges outwards. The degenerative cascade of the intervertebral disc initially involves desiccation of the nucleus pulposus. With decreased elasticity and dampening from the nucleus, increased loads are transmitted to the annulus and facets. The increased stress on the annulus can lead to fissures and radial tears in its collagen fibers. With further degeneration, this can lead to circumferential bulging of the disc, contained and uncontained disc herniations, and complete desiccation of the disc. This degenerative cascade can result in axial pain, by stimulating pain fibers in the annulus, or compression of spinal nerve roots and/or the spinal cord. This can manifest itself in motor weakness, pain and/or numbness in the arms or legs or both.
The structure and function of the discs may be altered by a variety of factors including repeated stress, trauma, infection, neoplasm, deformity, segmental instability and inflammatory conditions. Degeneration of the intervertebral disc is the most common etiology of clinical symptoms referable to the spine. Degeneration of the spine is a universal concomitant of human aging. In the cervical spine, neck and arm pain caused by nerve root compression has been estimated to affect 51% of the adult population. Spondylosis of the spine and aging are intimately related, with spondylosis increasing in both prevalence and severity with age. Fortunately, the majority of patients will improve without surgery. In approximately 10-15% of cases, spondylosis is associated with persistent nerve root and spinal cord compression and/or spinal pain, with a small percentage ultimately requiring surgery.
The most common type of surgery used in the United States for the treatment of degenerative disorders of the spine (spondylosis) is spinal fusion. In an interbody fusion, the diseased disc is removed and either a wedge of bone from the patient's hip, allograft or a metallic spacer is placed between the vertebrae where the disc was removed. This immobilizes the functional spinal unit. While this surgery has been successful in eliminating motion, there are disadvantages associated with it. By converting a mobile, functional spinal unit into a fixed, nonfunctional one, fusion results in increased strain patterns at levels adjacent to the fused segment. When a segment of the spine is fused, there is elimination of motion at the level of surgery. Therefore, the stresses that would normally be absorbed by the disc at the site of surgery are now transferred to adjacent segments. This can cause adjacent segment disease (ASD) to one or several spinal units adjacent to the affected level. ASD can be defined as a clinical syndrome of symptomatic degenerative changes occurring adjacent to a previously fused motion segment. Retrospective studies have estimated that ASD can occur in the cervical spine at a rate as high as 2.9% per year with a projected survivorship rate of 26% at 10 years (Hilibrand A S, Carlson G D, Palumbo M, Jones P K, Bohlman H H: Radiculopathy and myelopathy at segments adjacent to the site of a previous anterior cervical arthrodesis. J Bone Joint Surg (Am) 81:519-528, 1999).
In the cervical spine, thousands of North Americans undergo surgery for cervical spondylosis each year. The majority of these procedures involve an anterior discectomy with decompression of the spinal cord and/or nerve root. The primary indication for surgery in the management of cervical spondylosis is radiculopathy, myelopathy and/or neck pain. Following the discectomy, an anterior interbody fusion is commonly performed. Autologous bone harvested from the iliac crest or cadaveric bone is most commonly used to fill the space created by the removal of the disc. A number of other solutions have been suggested, including metallic devices such as fusion cages or other types of spacers, xenografts such as bovine bone, and biological strategies such as the use of growth factors. The graft for the interbody fusion can be shaped to correct underlying deformity of the cervical spine. By contouring the graft one can restore lordosis to a straight or kyphotic spine.
A more recent alternative to spinal fusion is replacement of the damaged disc with a motion preservation device, which includes either a nucleus or total disc replacement (TDR). The rationale for the development of the artificial disc is to prevent adjacent segment disease. Artificial disc devices can be broadly divided into two categories, those that replace the nucleus only, leaving the annulus and vertebral body end plates intact and those that involve replacement of the disc and addition of prosthetic end plates. Both strategies are directed at restoration of intervertebral disc function. Prosthetic nuclei are described, for example, in U.S. Pat. Nos. 5,047,055 and 5,192,326. United States Patent application US2002/0183848 also discloses a prosthetic spinal disc nucleus that has a hydrogel core surrounded by a constraining jacket.
There are several different types of prosthetic devices for use in the cervical or lumbar segments of the spine designed for TDR. For example, the Prodisc™ and the Charite™ disc are composites of cobalt chromium end plates with a polyethylene core. The Prodisc™ is described in U.S. Pat. No. 5,314,477 and the Charite™ disc is described in U.S. Pat. Nos. 5,401,269 and 5,556,431. The Prestige™ disc is another type of artificial disc that comprises a metal on metal design with a ball and trough articulation. Another type of artificial disc that is gaining popularity in the cervical spine is the Bryan® disc, described in several United States Patent applications including 2004/0098131; 2004/00544411; and 2002/0128715. The Bryan® disc is a composite artificial disc with a low friction, wear resistant, elastic nucleus that articulates with two circular metal plates.
Presently, there are at least four artificial cervical disc replacement systems undergoing clinical trials worldwide. These include unconstrained devices, such as the PCM cervical disc. These unconstrained devices do not have mechanical stops to limit their range of motion. The Bryan® Cervical disc, the Prodisc™ C and the Prestige™ LP cervical disc systems limit range of motion to varying degrees. These systems can be considered semi-constrained, in that there are mechanical stops outside the normal range of motion.
Artificial spinal discs have been implanted for the management of degenerative disc disease producing radiculopathy, myelopathy and/or axial spinal pain. More recently, artificial discs have been adopted for the treatment of trauma. The aim of TDR is to reproduce the biomechanics of the natural disc. Early clinical and biomechanical studies with single and multi-level disc replacement have reported favorable clinical outcomes and preserved range of motion at the level of surgery. Preservation of range of motion, however, while an important feature of an artificial disc, is only a single measure of spinal biomechanics. The effect of the disc on angulation at the operative level, the average disc space height, and overall spinal alignment (sagittal and coronal balance) also needs to be considered.
While the introduction of artificial discs has led to many successful surgeries, there are still problems associated with the current discs. For example, all of the current artificial cervical discs have a fixed height across the entire disc. The artificial discs presently available can have issues with focal kyphosis or kyphosis at adjacent segments of the spine after the patient post-operatively reassumes an upright position, supporting the weight of the head and body. For instance, with the Bryan® disc, the end plates are allowed to move freely about all axes of rotation, allowing the end plate to assume a position resulting from the forces exerted on the implant by the head and neck. At times, this position may be significantly different from the positioning of the disc intra-operatively. Several published studies with the Bryan® cervical disc replacement system have reported a tendency for the end plates of the prosthesis and the alignment of the cervical spine to develop kyphosis following surgery. [Pickett G E, Mitsis D K, Sekhon L H et al. Effects of a cervical disc prosthesis on segmental and cervical spine alignment. Neurosurg Focus 2004; 17(E5):30-35; Johnson J P, Lauryssen C, Cambron H O, et al. Sagittal alignment and the Bryan® cervical disc. Neurosurg Focus 2004; 17(E14):1-4; Sekhon L H S. Cervical arthroplasty in the management of spondylotic myelopathy: 18 month results. Neurosurg Focus 2004; 17(E8):55-61.] This kyphotic angulation of the prosthesis has been attributed to the passive (unconstrained motion with a mobile nucleus and variable instantaneous axis of rotation) design of the implant. None of the current TDR systems addresses this major complication.
A significant number of patients with spinal disc disease have a loss of sagittal alignment of the spine as a result of the degenerative process. In addition, varying degrees of coronal imbalance can also occur. None of the available artificial disc replacement systems are designed to restore normal alignment to a spine that is straight, which have focal/global kyphosis or coronal deformity. Existing artificial disc replacement systems that are inserted into either a straight, kyphotic or angulated segment are likely to take on the angle and local biomechanics determined by the facets, ligaments and muscle forces. As such, patients with a pre-operative straight spine may develop post-operative kyphosis, and patients with a pre-operative kyphosis may have a worsening of the deformity post-operatively. Kyphosis of the spine has been implicated in segmental instability and the development of clinically significant degenerative disease. Several clinical studies have described that a change in the sagittal or coronal balance of the spine can result in clinically significant axial spinal pain as well the initiation and/or the acceleration of ASD. [Kawakami M, Tamaki T, Yoshida M, et al. Axial symptoms and cervical alignment after anterior spinal fusion for patients with cervical myelopathy. J Spinal Disord 1999; 12:50-60; Harrison D D, Harrison D E, Janik T J, et al. Modeling of the sagittal cervical spine as a method to discriminate hypolordosis: results of elliptical and circular modeling in 72 asymptomatic subjects, 52 acute neck pain subjects, and 70 chronic neck pain subjects. Spine 2004; 29:2485-2492; Katsuura A, Hukuda S, Saruhashi Y, et al. Kyphotic malalignment after anterior cervical fusion is one of the factors promoting the degenerative process in adjacent intervertebral levels. Eur Spine J 2001; 10:320-324; Ferch R D, Shad A, Cadoux-Hudson T A, Teddy P J. Anterior correction of cervical kyphotic deformity: effects on myelopathy, neck pain, and sagittal alignment. J Neurosurg 2004; 100:S13-S19; Katsuura A, Hukuda S, Imanaka T, Miyamoto K, Kanemoto M. Anterior cervical plate used in degenerative disease can maintain cervical lordosis. J Spinal Disord 1996; 9:470-476.]
Attempting to provide a deformity correction by simply altering the end plate or the nucleus of an artificial disc, while still maintaining free movement about all axes of rotation, may not be sustainable as the forces exerted by the head and body on the artificial disc could counteract the desired correction. To provide a sustainable correction, some limitation on the axes of rotation is required. From a design perspective, the goal is to design an artificial disc that is able to correct deformity (coronal and sagittal), has mechanical stops outside the normal range of motion (semi-constrained), and preferably has variable instantaneous axis of rotation (IAR).
The limits on the axes of rotation can fall into two categories. One is to provide correction using a permanent rotation or translation of an axis to support the correction. This is accomplished using the geometries of the core and end plates themselves and is referred to the Geometric Constraint category. The second is to keep free range of motion about all axes but provide the correction using a material support. This type of design provides the correction by the imposition of a deformable material in the plane of correction for normal rotation in that plane. This is the Material Constraint category of designs.
Degenerative disc disease is a major source of morbidity in our society. It can lead to serious economic and emotional problems for those afflicted. Thus, there is a need for an artificial disc that can alleviate both symptoms and correct deformity (sagittal or coronal or both) of the spine.